The present invention relates to biphenyltetracarboxylic dianhydride and a process for producing the biphenyltetracarboxylic dianhydride, and more particularly, to high-purity biphenyltetracarboxylic dianhydride having much less contents of impurities and water which inhibit the production of high-molecular polyimide or polyamic acid from the biphenyltetracarboxylic dianhydride, and a process for stably producing the biphenyltetracarboxylic dianhydride.
Biphenyltetracarboxylic dianhydride (hereinafter occasionally referred to merely as “BPDA”) is a useful compound as a raw material for production of aromatic polyimides that have been noticed as heat-resistant resins. The aromatic polyimides can be produced from the raw BPDA by a method of polymerizing BPDA with aromatic diamines, a method of subjecting a polyamic acid obtained by polymerizing BPDA with an aromatic diamine at a low temperature near an ordinary temperature to ring-closing imidization reaction, or the like.
The BPDA has been generally produced by subjecting biphenyltetracarboxylic acid (hereinafter referred to merely as “BTC”) to dehydration ring-closing reaction. The BTC can be produced, for example, by the following methods (i) and (ii).
(i) Method of subjecting tetramethyl biphenyltetracarboxylate obtained by dehydrogenation dimerization reaction of dimethyl o-phthalate to hydrolysis in an aqueous medium in the presence of an acid catalyst.
(ii) Method of subjecting 4-halophthalic acid obtained by halogenating phthalic anhydride to dehalogenation dimerization reaction in an aqueous medium in the presence of an alkali, a reducing agent and a Pb catalyst to obtain an aqueous solution of a tetraalkaline metal salt of biphenyltetracarboxylic acid, and then neutralizing the tetraalkaline metal salt of biphenyltetracarboxylic acid with a mineral acid.
When BPDA containing impurities such as biphenyltricarboxylic anhydride is used to produce the above polymers, the increase in polymerization degree (viscosity) thereof is inhibited, thereby failing to produce high-molecular polyimide or polyamic acid. Accordingly, there have been proposed various methods of preventing discoloration, inclusion of fine insoluble particles as well as a trace amount of metals, or production of other impurities when the BPDA is produced by subjecting the BTC to dehydration ring-closing reaction.
For example, there has been proposed a method of heating BTC in a solid state to a temperature of 150 to 230° C. to subject the BTC to dehydration ring-closing reaction for obtaining BPDA, vaporizing the thus obtained BPDA by heating to a temperature of 250 to 400° C. under reduced pressure, and then cooling the vapor of the BPDA to recover the BPDA in the form of purified crystals (for example, refer to Japanese Patent Publication (KOKOKU) No. 4-37078).
However, the above method only may fail to prevent biphenyltricarboxylic anhydride (hereinafter referred to merely as “tri-compound”) as impurity to be mixed in the aimed product, thereby inhibiting the production of high-molecular polyimide or polyamic acid. The tri-compound tends to be mainly produced at the dehydration ring-closing reaction stage in which the BTC is heated to a temperature of 150 to 230° C. and at the temperature rise stage in which the BPDA is heated to a temperature of 250 to 400° C.
For the purpose of inhibiting production of the tri-compound, it has been attempted to remove water such as adhering water and crystal water from the BPDA. For example, there has been proposed a method of heating BPDA up to a temperature capable of removing water therefrom at a temperature rise rate of not more than 50° C./hr, and then successively heating the BPDA at a temperature of 250 to 300° C. for at least 3 hours to completely remove water from the BPDA (for example, Japanese Patent Application Laid-Open (KOKAI) No. 1-104063).
Alternatively, there has been proposed a method for producing BPDA by heat-treating BTC in which the BTC is contacted with an inert gas at a temperature of 250 to 300° C. for 8 to 40 hours to produce the BPDA while removing a tri-compound thereof as a by-product from a reaction mixture (for example, refer to Japanese Patent Publication (KOKOKU) No. 4-76991).
However, the above conventional methods have such a problem that even if the production of the tri-compound is prevented, the resultant aromatic polyimide may still fail to show a sufficiently enhanced viscosity (i.e., not improved in molecular weight). The reason therefor has been considered to be that BPDA is hydrolyzed by water absorbed therein to ring-open one of acid anhydride groups of BPDA, thereby producing biphenyltetracarboxylic monoanhydride (hereinafter referred to merely as “half compound”) which tends to exist as impurity in the resultant polymer. That is, the half compound having an anhydride group contributing to the polymerization and a carboxyl group not contributing thereto, tends to produce a polymerization-inhibiting terminal group like the tri-compound when producing the aromatic polyimide by polymerization of BPDA. However, there have been proposed no effective methods for preventing the production of the half compound.
Also, although it is considered that water absorbed in BPDA is present in the form of free water, crystal water or water for ring-opening the cyclic anhydride group of BPDA, the configuration of water contained in BPDA as well as the influence of the water on molecular weight of the resultant aromatic polyimide, etc, have not been clearly recognized. Therefore, at present, any effective water-removing methods for preventing the production of the half compound are still unknown.
Upon the production of the aromatic polyimide, BPDA is generally pulverized into fine particles before being subjected to the reaction. However, since BPDA has a moisture-absorbing property, the pulverization of BPDA tends to cause problems such as increase in water absorption due to increased surface area thereof. More specifically, when a reactor of a chemical industrial scale is used, the fine BPDA particles are blown up in a gas-phase portion of the reactor and frequently contacted with outside air therein, so that water present in the gas-phase portion tends to be absorbed in the fine BPDA particles, resulting in risk of production of the above half compound as impurity.
Therefore, in order to prevent undesirable water absorption in BPDA and produce a high-quality aromatic polyimide, an inside of the reactor used for producing the aromatic polyimide must be kept under excessively dried condition since drying conditions of the inside of the reactor are not clearly known, thereby causing problems such as increased industrial costs.